Sub-Micromolar Pulse Dipolar EPR Spectroscopy Reveals Increasing CuII -labelling of Double-Histidine Motifs with Lower Temperature

Abstract

Electron paramagnetic resonance (EPR) distance measurements are making increasingly important contributions to the studies of biomolecules by providing highly accurate geometric constraints. Combining double-histidine motifs with Cu^II spin labels can further increase the precision of distance measurements. It is also useful for proteins containing essential cysteines that can interfere with thiol-specific labelling. However, the non-covalent Cu^II coordination approach is vulnerable to low binding-affinity. Herein, dissociation constants (K_D ) are investigated directly from the modulation depths of relaxation-induced dipolar modulation enhancement (RIDME) EPR experiments. This reveals low- to sub-μm Cu^II K_D s under EPR distance measurement conditions at cryogenic temperatures. We show the feasibility of exploiting the double-histidine motif for EPR applications even at sub-μm protein concentrations in orthogonally labelled Cu^II -nitroxide systems using a commercial Q-band EPR instrument.

Keywords: EPR spectroscopy; RIDME; dissociation constant; double-histidine motif; non-covalent interactions.